1. Field of the Invention
The present invention relates to an oil pan structure and an engine therewith whereby the structure comprises: an oil pan flange; a bottom wall that forms a bottom part of an engine; two side walls (i.e. a left side wall and a right side wall) along a longitudinal direction of the engine, each wall that communicates a longitudinal edge of the bottom wall with the oil pan flange; and two side walls (i.e. a front side wall and a rear side wall) along a transverse direction of the engine, each wall that communicates a transverse edge of the bottom wall with the oil pan flange, thereby, the structure forms a boxy space, being capable of storing engine-internal oil therein.
2. Description of the Related Art
Conventionally known oil pan structures comprise: an oil pan flange that is fastened with bolts to the undersurface of the crankcase; a bottom wall that forms a bottom part of an engine; two side walls along a longitudinal direction of the engine, each wall that communicates a longitudinal edge of the bottom wall with the oil pan flange; and two side walls along a transverse direction of the engine, each wall that communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space so that the space can store engine-internal oil therein. In many cases, such an oil pan is manufactured by means of welding thin sheets of metal; thus, the stiffness of the structure is not sufficient. Further, the engine vibration level is high in general. Therefore, various measures to secure the strength as well as to control the vibration have been proposed.
A patent reference 1 (JP1999-280548) discloses an oil pan structure whereby the bottom part is reinforced with a bellow type plate, that is, with a plurality of reinforcement ribs 27, while the left- and right-side walls are provided with a plurality of reinforcement ribs 26, inside the oil pan.
A patent reference 2 (JP 2002-227626) discloses an oil pan structure whereby the bottom part is reinforced with a plurality of reinforcement channels 58, whereby the channels give the bottom part a bellow surface.
A patent reference 3 (JP2002-364324), i.e. a reference 4 (JP 2007 Patent 3925614) discloses an oil pan structure whereby the bottom part is provided with a plurality of reinforcement beads 1k′, while the left- and right-side walls, along a longitudinal direction of the engine, are provided with a plurality of reinforcement beads 1a′. 
A patent reference 5 (JP1999-270408) discloses an oil pan structure whereby the bottom part is provided with a plurality of reinforcement beads along the engine longitudinal direction as well as the engine transverse direction.
In an oil pan structure comprising: an oil pan flange that is fastened with bolts to the undersurface of the crankcase; a bottom wall that forms a bottom part of an engine; two side walls along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange; and two side walls along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space so that the space can store engine-internal oil therein,
the mentioned oil pan is, in many cases, manufactured by means of welding thin sheets of metal.
Thus, the stiffness of the structure is not sufficient; further, the engine vibration level is high in general. As the mentioned patent references 1 to 5 disclose, mainstream measures to secure the strength as well as to control the vibration are based on reinforcement beads which are provided on the bottom part of the oil pan, the left/right side wall along the engine longitudinal direction, or the front/rear side wall along the engine transverse direction.
However, in the mentioned measures according to conventional manners to provide an engine oil pan with reinforcement beads, improvements on strength as well as vibration-reduction are limited in spite of the reinforcement beads. The reason is that the beads are provided independently on the bottom part of the oil pan, on the left/right side wall along the engine longitudinal direction, or on the front/rear side wall along the engine longitudinal direction,                whereby the oil pan structure comprises: an oil pan flange that is fastened with bolts to the undersurface of the crankcase, a bottom wall that forms a bottom part of an engine, two side walls along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange, and two side walls along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space.        
Further, it is noted that conventional oil pans are often of a welded structure. The present invention, however, is based on the premise that the oil pan structure is basically a one-piece press-work product.
Moreover, it is noted that the mentioned conventional arts do not disclose how a natural frequency of an oil pan can be adjustable in connection with the shape of the reinforcement beads.
In this specification, a term “bead” is fundamentally used for explanation, so as to clarify that the oil pan walls are provided with corrugation reinforcement. Namely, this invention clearly recognizes a bead as a groove that is provided on a sheet metal by means of press working. A groove may forms a module of corrugation. A plurality of grooves may form continuation of corrugation.
Further, a closed end of a bead is defined as a groove end where the groove disappears toward the original sheet metal, with a trace curve of a U-shape, while an open end or end-opening of the bead means a groove end where the groove disappears toward the original sheet metal, without a trace curve. Sometimes, in this open end or end-opening, the groove disappears three-dimensionally with a shape of a round egg-cone. For this reason, a bead both ends of which are the open end or end-opening is named a substantially egg-shaped bead, in this specification. On the other hand, when one end of a bead is a closed end, the bead is named a substantially U-shaped bead in this specification.